Composition and method for coating stainless metal articles



United States Patent Ofi ice 3,121,033 Patented Feb. 11, 1964 3,121,033 COMPOSI'I'IUN AND IVL'ETHGD FOR CGATING STAINLESS METAL ARTICLES John M. Stapleton, Detroit, Mich, assignor to Detrex Chemical Industries, Inc., Detroit, Mich, a corporation of Michigan No Drawing. Filed Jan. 17, 1962, Ser. No. 166,937 6 Claims. (Cl. 148-624) This invention relates to an improved composition and method for forming tightly adherent coatings on metal articles which are to be subjected to plastic deformation. More particularly, it relates to the formation, on ferrous-based stainless metal articles of tightly adherent oxalate coatings on the surfaces thereof.

It is known to coat ferrous-based stainless metals such as stainless steel and the like with an adherent surface coating on an oxalate compound prior to plastically deforming such articles by such methods as drawing and starnping. However, all of the effective known prior art treatments require either the utilization of a complex pretreatment of the stainless surfaces, or the use in the treating bath of expensive and/or dangerous compounds.

For example, one prior art process involves a pre-treatment comprising an acid pickle and alkaline rinse. This incidentally requires the use of additional Water rinses, and in the event that the alkaline rinse contains the conventional alkali metal cyanide, a further Water rinse is required to obviate the liberation of hydrogen cyanide from the hot oxalate treating bath. Another prior art process discloses the use of a combination of a silicofiuoride compound and an organic nitro compound as activators or accelerators in an oxalate treating bath.

In a plant test, an oxalate coating deposited from a liquid bath including oxalic acid, sodium silicofluoride, sodium bisulfate, ferric sulfate and sodium thiosulfate, on to various types of stainless steel tubing was either poorly adherent to the stainless steel or did not deposit an oxalate coating having the characteristics necessary to perform satisfactorily during cold extrusion. It has now been found that the use of a silicofiuoride such as sodium silicofiuoride in an oxalate or oxalic acid bath, as above described, actually reduces the amount of coating deposited from such bath on various types of stainless steel including types 304, 310, 316, 3211 and 347. Furthermore, sodium silicofluoride influences the titration of total acid in such a bath, and since this titration is conducted by obtaining a known volume of the bath and adding an indicator such as phenolphthalein, followed by neutralization with sodium hydroxide of known normality, it becomes impossible to determine the actual, oxalic acid content due to the increase in titration caused by sodium silicofiuoride. Moreover, in such an oxalate bath containing sodium silicofluoride, normal exhaustion of the bath during the coating operation depletes the oxalic acid content of the bath, but does not, however, deplete the sodium silicofluoride content. Therefore, on replenishment with fresh chemical containing both oxalic acid and sodium silicofluoride there is a cumulative gain in the concentration of sodium silicofluoride in the bath.

It is therefore an object of this invention to provide an oxalate treating composition and method for use in the coating of stainless surfaces that is greatly simplified over the above prior art processes.

It is another object of this invention to provide an oxalate treating composition and method which effectively coats stainless surfaces without the necessity of pre-treatment modification of the surfaces to be treated.

It is another object of this invention to provide an oxalate treating composition and method which does not require the use of expensive or dangerous compounds.

It is another object of this invention to provide an oxalate coating composition and method which does not utilize a silicofluoride.

Other objects and advantages of the composition and method of this invention will readily become apparent from a reading of the following description and claims.

it has surprisingly been discovered that clean ferrousbased stainless metal surfaces may be effectively coated with an adherent oxalate coating, suitable for metal extrusion purposes, by contacting said surfaces with a single aqueous solution consisting essentially of oxalic acid, thiosulfate ions and at least one ion selected from the group consisting of ferric ions and bisulfate ions. Utilization of the composition and method of this invention results in the rapid formation on stainless metal surfaces of uniform densely crystalline adherent coatings.

Examples of aqueous baths suitable for use in accordance with the present invention may have an analysis falling within the following ranges (concentrations in grams per liter of water solution):

Oxalic acid .O2.05 Sodium bisulfate .0005.004 Ferric sulfate DOS-.009 Sodium thiosulfate .0006.002

A preferred bath composition has the following analysis (concentrations also in grams per liter of Water solution):

Oxalic acid .0375

Sodium bisulfate .0037 Ferric sulfate .0087 Sodium thiosulfate .008

pound of sodium thiosulfate for each gallons of solu tion is added to the bath. The stainless articles are then immersed in the bath for about 10 to 15 minutes while ensuring complete contact of the treating solution with all parts of the work. When the articles have received the desired coating density, they are removed from the bath, drained, rinsed in water, dried and lubricated.

Ferrous-based stainless articles treated in this manner are ready to be plastically deformed in the conventional manner.

During the treatment in accordance with the process of this invention the treating solution should be kept at a titration of from 30 to 50 points total acid when 5 ml.

of the solution is titrated with 0.1 N sodium hydroxide and phenolphthalein is used as the indicator. The acidity is adjusted by the addition of proper quantities of oxalic acid, ferric sulfate and sodium bisulfate. The iodine equivalent is maintained at from 1.0-2.0 ml. of 0.1 N iodine solution, per 25 ml. of solution by addition of sodium thiosulfate.

The following examples are illustrative of specific dry compositions which when made into water solutions with sodium thiosulfate are efiective in accordance with EX- ample 1.

Example 2 Percent by weight of dry composition Oxalic acid 70.00 Sodium bisulfate 6.00 Ferric sulfate 24.00

Example 3 Oxalic ari 80.00 Sodium bisulfate 5.00 Ferric sulfate 15.00

Example 4 Oxalic acid 65.00 Sodium bisulfate 8.00 Ferric sulfate 27.00

Example 5 ()xalic acid 85.00 Sodium bisulfate 5.00 Ferric sulfate 10.00

Example 6 Oxalic a 60.00 Ferric sulfate 40.00

Example 7 Oxalic acid 80.00 Ferric sulfate 20.00

Example 8 Oxalic acid 92.00 Sodium bisulfate 8.00

A preferred composition of dry ingredients has the following composition:

Having thus described my invention, I claim:

1. A process for treating the surface of ferrous-based stainless metals which comprises the step of contacting the clean metal surface with an aqueous bath consisting essentially of water, at least about .02 gram oxalic acid l per liter of solution, thiosulfate ion, and at least one ion selected from the group consisting of ferric ion andbisulfate ion, while maintaining said aqueous bath at a titration of from about 30 to 50 points total acid when 5 ml. of solution is titrated to a phenolphthalein end point with 0.1 N NaOH, and at an iodine equivalent of' from about 1.0 to 2.0 ml. of 0.1 N iodine solution per 7 25 ml. of solution, whereby there is formed on said surface a uniform dense adherent crystalline oxalate coating.

2. The process of claim 1 wherein said aqueous bath is maintained at a temperature in the range of from about room temperature to about 160 F.

3. The process of claim 1 wherein said aqueous bath is maintained at a temperature in the range of from about F. to F.

4. A process for treating the surface of ferrous-based stainless metals which comprises the steps of (1) preparing an aqueous solution consisting essentially of water, at least about .02 gram oxalic per liter of solution acid and at least one ion selected from the group consisting of ferric ion and bisulfate ion,

(2) adding to said solution a compound capable of furnishing thiosulfate ions and (3) then immersing in said solution a ferrous-based stainless metal article, whereby there is formed on' the surface of said article a uniform, dense, adherent,

crystalline oxalate coating, wherein said aqueous bath is maintained at a titration of from about 30 to 50 points total acid when 5 ml. of solution is titrated to a phenolphthalein end point with 0.1 N NaOI-l, and at an iodine equivalent of from about 1.0 to 2.0 ml. of 0.1 N iodine solution per 25 ml. of solution.

5. A liquid treating composition for producing a uniform, dense, adherent, crystalline oxalate coating on the surface of ferrous-based stainless articles, said composition comprising an aqueous solution consisting essentially of (1) oxalic acid'in an amount within the concentration range of from about .02 gram per liter to about .05

gram per liter (2) sodium thiosulfate in an amount within the range of from about .0006 gram per liter to about .002 gram per liter and at least one compound selected from the group consisting of (3) ferric sulfate in an amount within the concentration range of from about .005 gram per liter to about .009 gram per liter and (4) sodium bisulfate in an amount wtihin the concentration range of from about .0005 gram per liter to about .004 gram per liter.

6. The aqueous composition of claim 5 having the fol lowing composition:

Grams/liter Oxalic acid .0375 Sodium bisulfate .0037' Ferric sulfate .0087 Sodium thiosulfate .003

References Cited in the file of this patent UNITED STATES PATENTS Gibson Nov. 11, 1952 

1. A PROCESS FOR TREATING THE SURFACE OF FERROUS-BASED STAINLESS METALS WHICH COMPRISES THE STEP OF CONTACTING THE CLEAN METAL SURFACE WITH AN AQUEOUS BATH CONSISTING ESSENTIALLY OF WATER, AT LEAST ABOUT .02 GRAM OXALIC ACID PER LITER OF SOLUTION, THIOSULFATE ION, AND AT LEAST ONE ION SELECTED FROM THE GROUP CONSISTING OF FERRIC ION AND BISULFATE ION, WHILE MAINTAINING SAID AQUEOUS BATH AT A TITRATION OF FROM ABOUT 30 TO 50 POINTS TOTAL ACID WHEN 5 ML. OF SOLUTION IS TITRATED TO A PHENOLPHTHALEIN END POINT WITH 0.1 N NAOH, AND AT AN IODINE EQUIVALENT OF FROM ABOUT 1.0 TO 2.0 ML. OF 0.1 N IODINE SOLUTION PER 25 ML. OF SOLUTION, WHEREBY THERE IS FORMED ON SAID SURFACE A UNIFORM DENSE ADHERENT CRYSTALLINE OXALATE COATING. 